The present invention relates to a blow box for controlling or supporting the web run in a paper machine, particularly in the drying section of a paper machine, or in other corresponding devices, such as in a board machine, in a finishing machine and in coating machines.
The web run needs control or support, for instance during running in the area of the pockets formed between the drying cylinders in the drying section of a paper machine, particularly in such locations where the web needs to be released in a controlled manner from a drying cylinder and to run freely together with the wire to a turning roll, suction roll or the like.
In order to support the web at the pockets of the drying section it is known to use blow boxes, which eject air away from desired regions in order to create a negative pressure in these regions. Thus the negative pressure created by the blow boxes can be used to support the release of the web from a drying cylinder and to support the web run to a turning roll or the like.
It is known to arrange in a blow box, at the beginning and/or at the end of the negative pressure region created by the box, a blocking plate or the like, which projects towards the web and which is flexibly fastened to the blow box. The object of the blocking plate is to seal the negative pressure region from the surrounding space in order to maintain an as effective negative pressure as possible in the negative pressure region. The surface of the blocking plate, which is directed towards the web, can be protrudingly arched towards the web, with respect of the running direction of the web. The arched surface forms a so called Coanda surface, which facilitates the ejection of air away from the negative pressure region and prevents leaking air from entering the negative pressure region.
In this way, with the blow boxes in use, it can be created, with reasonable blow effects, intensified negative pressure regions having negative pressures of e.g. 0.1 to 0.4 kPa. However, as the paper machine speeds still rise and as the paper quality requirements increase, the order of the negative pressure level at particularly critical points should be even higher than 5 kPa.
However, the intensifying of the negative pressure level from the present, i.e. the maintaining of an even higher negative pressure with the aid of blowers, substantially increases the required blowing effect, in other words the energy costs. The higher the aimed negative pressure level, the larger are also the air leaks and their impacts on the energy costs. It is not possible to completely seal the negative pressure region from the surrounding space in order to reduce the leaks. Blocking members, blow nozzles or other structures of a blow box arranged too close to the wire can easily damage the wire, and they can themselves be easily damaged when the wire touches them. Thus, with the present devices there must be left a certain minimum gap between the blow boxes and the web's supporting wire in order to avoid damage to these members, to the web and/or to the wire in different running situations.
For instance a “paper lump” can push the wire to touch parts of the blow box, particularly the blow nozzles or blocking members, despite the minimum gap. In known blow boxes, a spring or some other mechanical member is used to keep nozzles projected towards the wire. The object is that the spring enables the nozzle to be pushed away from the wire, when required. However, springs are generally relatively stiff, and they are not sufficiently resilient in order to be able to adapt to all situations sufficiently rapidly. In addition, the spring force cannot be adjusted to different requirements. The spring must be subjected to a relatively high minimum pressure before it allows the nozzle to be pushed away from the wire.